st: Keeping specific observations

Dear Statalisters,
I would appreciate your help with the following conundrum. Assume I have a dataset that looks like this:
Year ID v1
2000 1 2
2001 1 2
2002 1 3
2000 2 3
2001 2 .
2002 2 3
2000 3 .
2001 3 .
2002 3 .
What I want to achieve is to only keep those individuals that appear with any non-missing values for v1 for any of the years in the dataset.
In other words, I don't think that a command: "keep if v1<." would do, because that would also take out the 2001 observation for the second individual (ID==2), which I'd like to remain in place.
Any ideas are welcome,
Eleimon
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